Fused bath electrolytic cells



Sept. 23, 1969 G. G. DAY

FUSED BATH ELECTROLYTIC CELLS Filed April 12, 1966 BY @me Gmo DAY United `States Patent O 3,468,786 FUSED BATH ELECTROLYTIC CELLS George Gerald Day, New York, N.Y., assignor to Chlormetals Incorporated, a corporation of Delaware Filed Apr. 12, 1966, Ser. No. 542,002 Int. Cl. C22tl 1/04 U.S. Cl. 204-243 Claims ABSTRACT 0F THE DISCLOSURE An improvement in the construction of horizontal electrolytic cells particularly adapted for the electrolysis of fused electrolyte salts floating on a owable molten metal cathode, the improvement relating to a bottom structure comprising a metal base and a relatively thick refractory overlayer, such refractory layer having at least one recess extending fully through its thickness so as to permit direct contact between the metal base and molten cathode. The refractory overlayer is one characterized by having a low coetiicient of heat transfer.

This invention relates to the construction of cells for the electrolysis at high temperatures of molten inorganic compounds in which a molten metal is used as a cathode. More particularly, this invention relates to a new design and construction of the bottom or sole of such cells.

In the construction of prior cells of this type, which are generally of a horizontal design, the bottom of the cell upon which the molten cathode rests or flows has heretofore been built of iron, steel, nickel, or other metal which would conduct the current from the molten cathode through the bottom to the bus bars leading to the current supply. Examples of these cells are the Acker cell (e.g., U.S. Patent No. 674,691), the Ashcroft cell (e.g., U.S. Patent No. 1,159,154), the Hamprecht cell used in Germany during World War Il and described in FIAT Report No. 830, and the early Szechtman cell as represented by U.S. Patent No. 3,104,213. In cells of this type, the bottom is necessarily subjected to substantially the same temperatures as that of the molten inorganic compound which is the electrolyte, generally temperatures in excess of 1300 F. and sometimes more than 1800 F. This is because the molten metal cathode which is sandwiched vbetween the bottom and the molten electrolyte is a good conductor of heat.

As a result of the bottom of the cell being subjected continuously to extremely high temperatures, the bottom members become mechanically weak and lose their dimensional stability. Also, at these high temperatures, most of the high temperature alloy steels are subject to attack by the molten metal. In addition, the electrical conductivity of the metals from which the cell bottoms have been made decreases with increased operating temperature. Finally, the high operating temperatures of the cell accelerate the formation of oxides in the molten cathode metal and also on the surface of the bottom, and the formation of these oxides causes corrosion and difiiculty in maintaining good electrical contact between the bottom and the molten cathode.

The cells to which my invention is particularly applicable are those adapted for the electrolytic decomposition at high temperatures of inorganic compounds and which comprise a substantially horizontal bottom surface, side walls, anodes in spaced relationship to and disposed above the bottom and above the cathode, and means separately electrically connected to the anodes and to the bottom and adapted to pass electrical current to and from them. Usually the cell bottom has a slight slope and the cell is designed to permit the molten metal cathode to flow across the bottom from one end of the cell to the other. These cells are adapted to receive the molten inorganic electrolyte which floats on the cathode and is in contact with the anodes and is decomposed by electrolysis during the operation of the cell.

I have now discovered that the deficiencies in the design and construction of former cells resulting from the bottom member being subjected to substantially the same temperature as exists in the molten inorganic compound may be remedied by making the bottom to comprise a metal base, preferably steel, and a relatively thick refractory overlayer disposed on a top of the base. The refractory overlayer is characterized by having a low coeicient of heat transfer and further characterized by having at least one recess extending fully through its thickness and permitting direct contact between the metal base and the molten metal cathode over the area beneath the recess. Normally a plurality of recesses are used, and their total cross-sectional area will be in the range of about 5% to 20% of that portion of the base that lies under the molten electrolyte. In cross-section, the recesses may be any shape, such as rectangular or circular. 0ptionally, the metal studs project up into the recesses from the base.

Specic embodiments of the improved electrolytic cell structure of this invention are shown in the annexed drawing, in which:

FIGURE 1 is a modified elevational side View taken perpendicular to the longitudinal axis of a cell;

FIGURE 2 is a plan view of the refractory overlayer having slot recesses, and is shown without any electrolyte or molten cathode;

FIGURE 3 is a plan view of the refractory overlayer depicting an alternative embodiment of the recesses; and

FIGURE 4 is a plan view of a refractory overlayer having cylindrical recesses.

FIGURES 2, 3 and 4 do not show electrolyte or molten cathode.

Referring to FIGURES 1 and 2, the horizontal electrolytic cell 11 is provided with a steel base member 12,

one or more anodes 13 and electrical connections 14, 15`

attached to the base 12 and anode 13. A relatively thick refractory layer 16 is disposed on the base 12 and has been cast to form the side walls 17 of the cell as well as to cover the base 12. A molten cathode such as molten lead 18 covers the surface of the refractory overlayer 16 between the side walls and above the base, and the molten electrolytic 19 floats on top of the molten cathode.

The refractory overlayer 16 of FIGURES 1 and 2 is provided with slotted recesses 20 of rectangular crosssection which extend throughout the thickness of overlayer, and extend horizontally for most but not all of the width of the refractory overlayer beneath the molten electrolyte. The embodiment shown in FIGURES 1 and 2 includes two forms of studs projecting upwards from base 12 into the recess; an inverted U-shaped stud 21 is shown on the right, and inverted L-shaped stud 22 is shown on the left.

FIGURE 3 depicts an alternative embodiment in which recesses 23 are shorter with semi-circular ends and in which some recesses are disposed at right angles to other recesses. The studs 24 are cylindrical.

FIGURE 4 depicts a refractory overlayer which has circular recesses 25 and again cylindrical studs 24. The recesses 25 are set on a square pattern.

The refractory overlayer will be relatively thick, compared to the thickness at the bottom, and should have sufiicient thickness to reduce the temperature of the bottom plate during the cells operation to a temperature at which the plate maintains dimensional stability, and preferably to the range of 300 12.-600 F. The exact thickness used will depend upon the coefficient of heat transfer Aof the particular refractory used as well as upon the intended operating temperature of the molten electrolyte.

The recess or recesses in the refractory overlayer extend throughout its thickness, thereby permitting the molten cathode to iill the recess and to make electrical contact with the base. The recesses may be of any crosssectional shape, but are advantageously parallel slots with vertical sides or cylindrical holes. If desired, the sides of the recesses may be slanted in or out.

At least one and generally several recesses will be provided in the refractory overlayer for each cell. The total cross-sectional area of the recesses at the plane of the base depends upon the current density for which the cell is designed and is usually designed to be as small as is feasible consistent with other design factors. Such areas may vary from about 5% to about 20%, normally in the range of about to 12%, of the arca of the base through which current is passed. Although a large number of recesses having individually relatively small cross-sectional areas may be used, it is preferred to use a smaller number of recesses having a width or diameter of one to two inches and, illustratively, a length of 18-20 inches.

The refractory overlayer must have .a relatively low coeiicient of heat transfer in order to protect the base from excessively high temperatures. Suitable refractories include those having a high percentage of alumina or magnesia. It is desirable that the refractory be castable and somewhat porous. A cement-type refractory composition may suitably use calcium aluminate as a binder. In some uses, such as when decomposing a sodium compound, the silica content of a refractory should be minimized, because sodium tends to reduce the silica. A refractory overlayer may also suitably include a filler such as vermiculite or crushed tirebrick.

Studs may optionally be provided to assure adequate electrical contact and to reduce somewhat the required minimum cross-sectional area of the recesses. These studs are advantageous when the molten cathode shrinks, upon solidifying, as does lead, but they are not essential.

The electrolytic cells for which this invention is useful are used to decompose various molten inorganic compounds especially compounds of the alkali metals and the alkaline earth metals, eg., their halides, carbonates, nitrates, `sulfates and oxides, and especially the halides of sodium, potassium, magnesium and calcium.

In operating an electrolytic cell constructed as herein described, the necessary current passes from the anode 13, through the electrolyte 19, and then through the body of the cathode and through the studs, if any, into the base 12 at the point where the latter connects the former. When decomposing sodium chloride, the temperature of the molten salt is about 1550 F., and the temperature of the molten lead cathode is about the same. Normally, the temperature of base 21 would be at substantially the same temperature. However, because the refractory overlayer 16 covers most (80-95%) of the area of the base 12 and hence protects that portion of it from the high temperature, the primary heating of the base occurs only where the molten cathode is in direct contact with the base. Since the contact area is a small percentage of the overall area of the base, the heat transferred at contact area is readily dissipated, thus permitting operation of the cell with the electrolyte at a high temperature, while maintaining the base at a much lower average temperature without the use of cooling coils or other cooling means.

Having thus described my invention, I claim:

1. In a horizontal cell adapted for the electrolytic decomposition .at high temperatures of inorganic compounds and comprising a substantially horizontal bottom surface, side walls, anodes in spaced relationship to and disposed above said bottom and means separately electrically connected to said `anodes and to said bottom and adapted to pass electrical current to and from them, said cell being adapted to receive a flowable molten metal cathode disposed on said bottom and a molten inorganic compound as an electrolyte floating on said cathode and in contact with said anodes, the improved bottom structure which comprises a metal base and a relatively thick refractory overlayer disposed on top yof said base, said refractory overlayer having at least one recess extending fully through its thickness and permitting direct contact between said metal base and said molten cathode, and being characterized by having a low coefficient of heat transfer, said flowable molten metal cathode -owing substantially over said refractory overlayer.

2. The bottom structure of claim 1 wherein said refractory overlayer comprises predominantly alumina with a calcium aluminate binder.

3. The bottom structure of claim 1 wherein ksaid overlayer comprises a cast refractory mass, and wherein said recesses comprise a plurality of parallel slots.

4. The bottom structure of claim 1 wherein said overlayer is provided with substantially cylindrical recesses.

5. The improved structure of claim 1 wherein at least one stud protrudes into at least one recess for a distance not exceeding the thickness of said overlayer adjacent to said recess, said stud being characterized by having a high electrical conductance.

References Cited UNITED STATES PATENTS 895,159 8/1908 Carrier 204-219 XR 1,338,279 4/1920 Blumenberg 204-247 XR 1,563,187 11/1925 Harvey 204-243 XR 3,267,183 8/ 1966 Feinleib 204--243 XR FOREIGN PATENTS 493,602 6/ 1953 Canada.

JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner Us. C1. X.R. 204-219, 250 

